1. Field of the Invention
The invention relates to a lithium-ion battery, and more particularly to a basic unit of lithium-ion battery, a battery pack comprising the same, and a charge/discharge equalizing method thereof.
2. Description of the Related Art
Conventionally, a lithium-ion cell cannot have exactly the same voltage, current, and charge/discharge time as another lithium-ion cell, even if they have the same specifications (e.g., single stage) under production, charge/discharge process, and different application environments. Such nonuniformity in a battery pack composed of multiple lithium-ion cells shortens its charge/discharge time, with some lithium-ion cells failing to fully charge or output electrical energy. Thus, the battery pack will suffer from a low efficiency and a shortened lifespan. For example, an individual lithium-ion cell can be recharged 2000 times, but a battery pack composed of 12 lithium-ion cells can only be recharged 400 times. This is a fundamental reason why the capacity and the lifespan of a lithium-ion battery pack cannot be increased by increasing the number of the lithium-ion cells.
To solve the above problems, the following two types of charge equalizing methods are applied to lithium-ion batteries:
1. Equalizing Charge Method of Energy Consumption Type
While charging, excess electrical energy of a lithium-ion cell is consumed by using a shunt resistor connected in parallel.
However, such a method can only achieve a one-way equalizing while charging, but not during the discharge. It is also consumes energy and produces a large amount of heat. If the ventilation is insufficient, it may cause a lithium-ion battery to explode due to high temperatures and internal resistance.
Such a method cannot equalize high currents. For example, to prevent the heat of a resistor from affecting a lithium-ion battery, a 100 mA current is often selected to equalize, but the equalizing effect is unnoticeable, the equalizing control is complicated, and thus the lithium-ion cells cannot be packed together. Therefore, the equalizing charge method of energy consumption type is only used in fields with no safety requirements, as in airplane models, whose capacity is limited to a three-stage series-connected battery pack.
2. Equalizing Charge Method of Non-energy Consumption Type
Two ends of a lithium-ion cell are connected to two corresponding ends of an inductance (or capacitance), which is used as an energy transfer carrier. When a lithium-ion cell has redundant energy, a switch is turned on to connect and transfer the redundant energy to the inductance (or capacitance) and from there to a corresponding inductance (or capacitance) of a low-energy lithium-ion cell. Charge equalizing of the lithium-ion battery is achieved after the energy stored in the inductance (or capacitance) is transferred to the low-charge lithium-ion cell.
However, this method has the same disadvantages as one-way equalizing during the charge rather than during the discharge, and the control system is complicated. Since energy is consumed through transfers and large-sized inductances or capacitances occupy a large amount of space, high currents cannot be equalized, and thus battery packs with large capacity and high output current/voltage cannot be built.
In practical applications, lithium-ion battery packs without an equalizing device are mainly used in electric bicycles for civilian use and electric tools. The voltage of a lithium-ion battery pack is generally 24-36 V. If the battery pack is for an electric automobile, it must have a capacity (e.g. 480 V, 150 A) of 40-60 kW (equal to a displacement of 1.3-1.6 L).
Under such a capacity, if the limit of the overcharge voltage for an individual lithium-ion cell is 4.2 V and the limit of the overdischarge voltage is 3.0 V, when a battery pack composed of the lithium-ion cells is used for a certain number of charge/discharge cycles, the voltage difference between lithium-ion cells with the best and the worst electrochemical performance can be as high as 3.3-3.8 V (i.e. 0.5 V).
While charging, the worst lithium-ion battery can quickly reach the limit of the overcharge voltage and terminate the charge process; while in the discharge process, the worst lithium-ion battery can quickly reach the limit of the overdischarge voltage and terminate the discharge process.
Consequently, the lithium-ion battery pack has very low charge/discharge efficiency, especially for cases of greater numbers of series-connected battery packs, larger capacities, and longer charge/discharge times. Moreover, the overcharge and overdischarge management of the lithium-ion battery must be ensured, or the lifespan of the lithium-ion battery pack will progressively decrease.
The nonuniformity of lithium-ion cells is determined by their electrochemical properties and is a fundamental reason why multiple lithium-ion cells cannot be packed together.